Tunnel emitter photocathode

ABSTRACT

A method of producing a tunnel emitter photocathode consisting of heating a semiconductor layer and then depositing a layer of aluminum oxide on one side thereof at a rapid rate and then baking out the wafer in a hydrogen gas atmosphere. After depositing electrical contacts on each side of the wafer, a metallic emitter layer is evaporated over the aluminum oxide layer with the metallic emitter layer treated with a low work function material such as cesium and oxygen to further increase the emission efficiency.

United States Patent [191 [111 3,913,218 Miller Oct. 21, 1975 [54] TUNNEL EMITTER PHOTOCATHODE 3,795,977 3/1974 Berkenblit 29/585 Inventor: Brian S. Miller, Alexandria, Va.

Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

Filed: June 4, 1974 Appl. No.: 476,248

U.S. Cl. 29/585; 29/590; 357/30 Int. Cl. B01J 17/00 Field of Search 29/484, 485, 486, 590

References Cited UNITED STATES PATENTS l/l969 Okumura 29/584 10/1969 Ono 29/585 8/1973 Antula 29/584 Primary ExaminerW. Tupman Attorney, Agent, or FirmMax L. Harwell; Nathan Edelberg; Robert P. Gibson [57] ABSTRACT A method of producing a tunnel emitter photocathode consisting of heating a semiconductor layer and then depositing a layer of aluminum oxide on one side thereof at a rapid rate and then baking out the wafer in a hydrogen gas atmosphere. After depositing electrical contacts on each side of the'wafer, a metallic emitter layer is evaporated over the aluminum oxide layer with the metallic emitter layer treated with a low work function material such as cesium and oxygen to further increase the emission efficiency.

8 Claims, 1 Drawing Figure U.S. Patent Oct. 21, 1975 TUNNEL EMITTER PHOTOCATHODE The invention described herein 'may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND OF INVENTION The present invention is in the field of cold cathode tunnel emitter devices that convert light energy to photoelectrons for light imaging, intensifying, detection, or control purposes.

Although the principle of such devices has been known for some time, useful device characteristics have not been obtained due principally to the loss of electrons in the insulator and emitter layers during the V tunneling and emitting process and also a tendency for rapid breakdown and shorting of the insulator layer at the high electric fields required thereacross to obtain high tunneling efficiency.

SUMMARY OF INVENTION BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a cross-section of the tunnel emitter photocathode of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In reference to the FIGURE, the cross-sectioned tunnel emitter photocathode of the present invention is shown schematically in the environment which it operates. This device has been demonstrated to eject a higher number of electrons out the low work function material 18 toward collector 24 than in prior art tunnel emitters.

The tunnel emitter photocathode produced by the present method has a semiconductor layer 12 to detect light radiation thereon and an insulating layer 14 contiguous therewith on one side. A thin metallic emitter layer 16 is contiguous with insulating layer 14, and a low work function material layer 18, such as alternate layers of cesium and oxygen, is deposited on layer 16. A first bias voltage source 20 is connected to layers 12 and 16 such that electrons generated in the semiconductor layer 12 by photons impinging thereon are tunnelled through insulating layer 14 into the thin metallic emitter layer 16. A second bias voltage source 22 provides an electron acceleration bias on collector 24 that accelerates electrons from low work function layer 18 through the vacuum between layer 18 and collector 24.

Light radiation from either side, i.e. directly onto layer 12 or through layers 18, 16, and 14 and then onto layer 6 l2, creates electron hole pairs in layer 12.

The method of producing this tunnel emitter photocathode is as follows. The material of semiconductors layer 12 is the type used in previous photocathodes. Layer 12 may be a convenient material for detecting light radiation, such as silicon, but may be other materials that are suitable for detecting specific wavelengths, such as at infrared. The silicon is p-type material that has a resistivity of 10 ohm-centimeters. A particular combination of insulating material 14, the treatment of this insulating material, and the emitting layer are explained hereinbelow.

An amorphous layer of aluminum oxide (A1 0 represented by numeral 14, is deposited on the semiconductor layer 12 by a chemical vapor deposition process using aluminum trimethyl and oxygen in a chamber suitable for heating the semiconductor layer 12 to about 450 Centigrade. The aluminum trimethyl and oxygen, which are both gases, are diluted with a high volume of nitrogen gas, which is used as on inert carrier gas. The flow rate of this gas mixture into the chamber is adjusted to give a rapid rate of oxide deposition on layer 12. Typically, a deposit rate of about 2,000 A thickness per minute is used with a thickness of between 500 and 1,000 A being deposited. The resulting wafer that includes the insulating layer 14 of aluminum oxide on the semiconductor layer 12 is placed in a furnace having a hydrogen gas atmosphere and is typically heated to 650 Centigrade for 18 hours. After this treatment, the wafer is placed in a vacuum environment and suitable electrical contacts are deposited by evaporation of metal on layer 12 and the outer portion of layer 14. An emitter layer 16 is then deposited over the electrical contact on the outer portion of layer 14 by evaporating thereon in an ultra high vacuum system. This emitter layer 16 is a metal, such as silver and is integral with the electrical contact. Layer 16 is optically semitransparent and is about to 500 A thick. A low work function layer 18 is deposited on layer 16. Layer 18 may be alternate layers of cesium and oxygen that are deposited in the standard manner for photocathode activation to further increase the electron emission efficiency.

The tunnel emitter photocathode that is produced by the above method functions :in the environment as shown by the FIGURE. That is, a first bias voltage source 20 with polarity as shown is connected to the electrical contacts connected to layers 12 and 16 and has a forward bias of about 10" volts per square centimeter of the common area between layers 12 and 14. A second bias voltage source 22, of a few hundred volts value, is applied to collector 24 to accelerate electrons from layer 18 through the vacuum environment between low work function layer 18 and collector 24.

While only one embodiment of the invention has been disclosed, it is to be understood that variations in the details of fabrication, the materials used and the combination and arrangement of elements may be made while remaining within the spirit and scope of the invention which is limited only by the following claims.

I claim:

1. A method of producing a tunnel emitter photocathode, the steps comprising:

providing a thin layer of semiconductor material;

depositing a thin insulating layer on one side of said semiconductor material at a fast rate;

baking the wafer resulting from the semiconductor and insulating layers in a hydrogen gas atmosphere for an extensive period;

placing said wafer in a vacuum environment and evaporating electrical contacts on the outside of each of said semiconductor and insulating layers;

depositing a thin metallic emitter layer over said electrical contacts that is contiguous with said insulating layer;

treating said metallic emitter layer with a low work function material; and

providing a first bias voltage source to said electrical contacts to produce an electric field across said insulating layer between said semiconductor and said metallic emitter layer for tunneling electrons generated in said semiconductor through said insulating layer into said metallic emitter layer and out said low work function material.

2. A method of producing a tunnel emitter photocathode as set forth in claim 1 wherein said layer of semiconductor material is silicon that has a resistivity of ohm centimeters.

3. A method of producing a tunnel emitter photocathode as set forth in claim 2 wherein the step of depositing a thin insulating layer comprises heating said layer of semiconductor to 450 Centigrade and applying an amorphous layer of aluminum oxide that is deposited by chemical vapor deposition process using aluminum trimethyl and oxygen gases in a chamber that is diluted with a high volume of nitrogen gas as an inert carrier and providing a rapid rate of oxide deposition by having a fast flow rate for said gas mixture.

4. A method of producing a tunnel emitter photocathode as set forth in claim 3 wherein said rapid rate of oxide deposition is about 2,000 A per minute and a thickness of between 500 A and 1,000 A of oxide is deposited.

5. A method of producing a tunnel emitter photocathode as set forth in claim 4 wherein the step of baking the wafer of semiconductor and insulating layers in a hydrogen gas atmosphere is for 18 hours at a temperature of 650 Centigrade.

6. A method of producing a tunnel emitter photocathode as set forth in claim 5 wherein the step of depositing a thin metallic emitter layer is by evaporating silver to between A and 500 A thickness onto said insulating layer in an ultra high vacuum system.

7. A method of producing a tunnel emitter photocathode as set forth in claim 6 wherein the step of treating said metallic emitter layer with a low work function material is by application of cesium oxide.

8. A method of producing a tunnel emitter photocathode as set forth in claim 6 wherein the step of providing a first bias voltage source has a value of 10 volts per centimeter of common surface areas between the 

1. A method of producing a tunnel emitter photocathode, the steps comprising: providing a thin layer of semiconductor material; depositing a thin insulating layer on one side of said semiconductor material at a fast rate; baking the wafer resulting from the semiconductor and insulating layers in a hydrogen gas atmosphere for an extensive period; placing said wafer in a vacuum environment and evaporating electrical contacts on the outside of each of said semiconductor and insulating layers; depositing a thin metallic emitter layer over said electrical contacts that is contiguous with said insulating layer; treating said metallic emitter layer with a low work function material; and providing a first bias voltage source to said electrical contacts to proDuce an electric field across said insulating layer between said semiconductor and said metallic emitter layer for tunneling electrons generated in said semiconductor through said insulating layer into said metallic emitter layer and out said low work function material.
 2. A method of producing a tunnel emitter photocathode as set forth in claim 1 wherein said layer of semiconductor material is silicon that has a resistivity of 10 ohm centimeters.
 3. A method of producing a tunnel emitter photocathode as set forth in claim 2 wherein the step of depositing a thin insulating layer comprises heating said layer of semiconductor to 450* Centigrade and applying an amorphous layer of aluminum oxide that is deposited by chemical vapor deposition process using aluminum trimethyl and oxygen gases in a chamber that is diluted with a high volume of nitrogen gas as an inert carrier and providing a rapid rate of oxide deposition by having a fast flow rate for said gas mixture.
 4. A method of producing a tunnel emitter photocathode as set forth in claim 3 wherein said rapid rate of oxide deposition is about 2,000 A per minute and a thickness of between 500 A and 1, 000 A of oxide is deposited.
 5. A method of producing a tunnel emitter photocathode as set forth in claim 4 wherein the step of baking the wafer of semiconductor and insulating layers in a hydrogen gas atmosphere is for 18 hours at a temperature of 650* Centigrade.
 6. A method of producing a tunnel emitter photocathode as set forth in claim 5 wherein the step of depositing a thin metallic emitter layer is by evaporating silver to between 100 A and 500 A thickness onto said insulating layer in an ultra high vacuum system.
 7. A method of producing a tunnel emitter photocathode as set forth in claim 6 wherein the step of treating said metallic emitter layer with a low work function material is by application of cesium oxide.
 8. A method of producing a tunnel emitter photocathode as set forth in claim 6 wherein the step of providing a first bias voltage source has a value of 107 volts per centimeter of common surface areas between the insulating layer and said semiconductor material. 